BrainPy-style Synaptic Outputs

Synaptic output models for converting conductances to currents.

SynOut	Base class for synaptic output modules.
COBA	Conductance-based (COBA) synaptic output.
CUBA	Current-based (CUBA) synaptic output.
MgBlock	Synaptic output with voltage-dependent magnesium (Mg2+) block.